1. Field of the Invention
This invention relates to a process for the operation of a polymerization reactor for producing polymers. More particularly, this invention relates to a method for the control of monomer concentration in a polymerization process to provide improved control of at least one of polymer production rate, polymer melt index, and density control.
2. Description of the Prior Art
Although, for sake of clarity and brevity, this invention will be described in respect of the slurry phase polymerization of ethylene to produce high density polyethylene (HDPE), it is to be understood that this invention applies generally to polymerization processes in which a process fluid desirably flows in a substantially uniform manner, and in which a densitometer is employed. For example, this invention can be applied in polymerization systems wherein either slurry or solution polymerization of at least one monomer takes place.
Heretofore, HDPE has been formed by polymerizing ethylene while dissolved in a solvent such as isobutane using a silica-supported chromium/aluminum catalyst. Tri-ethyl borane (TEB) has been employed for various reasons, one of which was to form hexene, a co-monomer, in situ in the reaction mixture. Ethylene and molecular hydrogen (hydrogen) are added to form a final reaction mixture. This reaction mixture is circulated in a continuous stream (loop) in the reactor system, and a slurry is formed which is composed of this mixture and suspended solid polyethylene particles (powder). A slip stream of this slurry is withdrawn and solid polyethylene product recovered therefrom. The reaction is carried out at a temperature of from about 205 to about 225 degrees Fahrenheit (F.) and a pressure of from about 600 to about 700 psig inside a loop-type reactor that can be, for example, about 24″ in inside diameter, and about 728 feet long. Inside the reactor, the reaction slurry is circulated at a high velocity, e.g., about 35 feet per second, to prevent settling out of the polymer particles in the reactor.
The reaction product is withdrawn from the reactor as a slurry of polyethylene powder in liquid isobutane. In order to reduce the amount of isobutane that must be recycled through the purification section, the slurry is concentrated by the use of hydroclones after it leaves the reactor. Hot recycle water is added to the polymer slurry coming out of the hydroclones, and the combined streams flow into a high-pressure slurry flash drum where the isobutane and unreacted ethylene are removed overhead from the top of the drum, and the water and polymer are removed from the bottom of the same vessel. A slip stream taken from the over head gas from the high pressure separator is taken to a conventional gas chromatograph where a sample of the gas is periodically analyzed. This periodic analysis takes from 2 to 3 minutes per analysis cycle and typically has a 7 to 10 minute delay from real time. The composition of this gas sample gives an indication of the actual concentration of ethylene gas inside the slurry loop reactor.
Typically, the ethylene concentration is controlled to about 6 percent by mole (mole %), but, depending on the type of product made, it can be in the range from about 2 to about 8 mole %. The TEB concentration is controlled in the range from about 0.4 to about 0.7 ppm. Hydrogen is used to control chain branching. Typically, the hydrogen concentration in the reactor is in the range from about 0.80 to 1.2 mole %.
Processing conditions in the reactor can be varied to influence the polymers melt index, molecular weight distribution, and density. Temperature is an important variable in the polymerization process. Depending on the type of polymer resin made, reactor temperature is maintained at the desired level by circulating tempered water through jackets carried by the reactor. Around the reactor loop, a number of conventional thermocouples or resistance temperature detectors (RTD's) are employed for measurement of the temperature of the reaction mixture. Reactor temperature is maintained at the desired level, in part, by circulating tempered water through the reactor's jackets. The amount of ethylene fed to the reactor directly affects the temperature of the reaction.
In the reactor loop there is a conventional analyzer that measures the density of the slurry circulating in the reactor. This instrument continuously measures the density of the slurry which is indicative of the polymer solids concentration in the reactor. The solids concentration is typically maintained at a desired concentration, e.g., from about 37 to about 44% by weight (wt. %). This concentration can be controlled by adjusting the isobutane feed rate to the reactor.
The combined stream of reactor hydroclone bottom flow and hot recycle water is flashed into a high pressure slurry drum that is maintained at a pressure of from about 220 to about 230 psig. Most of the hydrocarbons are vaporized by the hot water and are recovered from the overhead stream of the drum by way of a cyclone separator. This cyclone separates and removes polymer particles from the overhead gas stream. Agitators hold the solid polymer particles in suspension in the water. The bottoms output of the high pressure drum is sent to a low pressure slurry drum which is maintained at a pressure below that of the high pressure slurry drum, e.g., about 1.5 psig. The slurry is thickened in the low pressure slurry drum, and then removed from the agitated section of the drum and pumped to centrifuges. Water from the centrifuges is discharged to the recycle water drum and then pumped to the recycle water separator. A recycle water separator is used to provide residence time to allow finely divided polymer powder fines to disengage from the recycle water. Solid polymer particles from the centrifuges are passed into a fluid bed dryer. After the fluid bed dryer, the dried polymer is conveyed to powder storage silos or a mixer feed hopper.
The melt index of the polymer in the reactor is controlled mainly by the reaction temperature and ethylene concentration in the reaction mixture. Polymer density is controlled by the concentration of the TEB and/or hexene present in the polymerization reaction mixture.
Process variables in the reactor can change suddenly and their effect on the ethylene concentration in the reactor may not be picked-up by the aforesaid gas chromatograph since that chromatograph has a 7 to 10 minute dead time (feedback delay). This delayed ethylene concentration analysis has a good probability from time to time of causing less than desired reactor production and polymer property control, especially during un-steady state processing conditions. Accordingly, there is a need for better monomer concentration analysis and control in polymerization processes such as the HDPE process aforesaid.
Pursuant to this invention, real time, on-line prediction and control of monomer concentration inside a polymerization reactor is substantially improved by using certain process instrumentation coupled with mathematical models to provide more consistent reactor control. Pursuant to this invention, a system that employs a densitometer measurement and a mathematical model for monomer concentration prediction reduces the aforesaid time delay by several orders of magnitude, and provides a more optimal monomer feed rate control.